1. Field of the Invention
This invention relates to an objective lens driver in an optical disk apparatus for driving an objective lens for an optical disk in a focusing direction and a tracking direction.
2. Related Background Art
Generally, an objective lens driver in an optical disk apparatus has two kinds of coils and a magnetic circuit for moving an objective lens in a focusing direction and a tracking direction, and one more kind of coil and a magnetic circuit, i.e, a linear motor, for causing the objective lens to seek any position in the radial direction of an optical disk, in order to effect the recording, reproduction and erasing of information on the optical disk. In recent years, there has been proposed an objective lens driver in which, for the purpose of shortening access time, an actuator for driving in the tracking direction and a linear motor are made common and further, a magnetic circuit for driving in the tracking direction is made common to that of the linear motor to thereby reduce the gross weight and simplify the driving portion.
FIGS. 1 to 3 of the accompanying drawings show an example of an objective lens driver according to the prior art in which a magnetic circuit is common to a driving portion in the focusing direction and a linear motor, FIG. 1 being a plan view, FIG. 2 being a cross-sectional view taken along line A--A of FIG. 1, and FIG. 3 being a cross-sectional view taken along line B--B of FIG. 1. Two parallel rails 1 and 1 are horizontally disposed on a base, not shown, and outer yokes 2 and 2, having a permanent magnets 3 and 3 attached to the inner sides thereof, and inner yokes 4 and 4 spaced slightly apart from the permanent magnets are fixed parallel and symmetrically in succession from the outer side between the two rails 1 and 1. The outer yokes 2 and the inner yokes 4 are equal in length to each other, and are connected together at their opposite ends by relay yokes 5. The permanent magnets 3 are shorter in length than the spacing between the relay yokes 5 and 5 at the two ends of an outer yoke 2, and are spaced apart from both of the relay yokes 5 and 5 at the two ends of an outer yoke 2. A carriage 6 is positioned between the rails 1 and 1 and is supported on the rails 1 and 1 through bearings 7a and 7b, and are movable in a direction parallel to the rails 1 and 1, i.e., the tracking direction. Coils 8 and 8 are fixed to the carriage 6, and the yokes 2 and the permanent magnets 3 pass through these coils. Vertically parallel leaf springs 9 and 9 are forwardly mounted on the central portion of the carriage 6 and vertically movably support a lens holding member 10 forward of the carriage 6. The lens holding member 10 is annular and the inner yokes 4 and 4 pass therethrough, and the lens holding member 10 holds an objective lens 11 on the center of the upper surface thereof with the optical axis of the objective lens kept vertical, and coils 12 and 12 are provided from the upper portions of the sides thereof to the end of the upper surface thereof and from the lower portions of the sides thereof to the end of the lower surface thereof. A portion of the carriage 6 is extended to the center of the lens holding member 10 and holds a 45.degree. upwardly facing mirror 13 just beneath the objective lens 11.
With such a construction, when an electric current is supplied to the coils 8, an electromagnetic force acts between the magnetic field between the permanent magnet 3 and the inner yoke 4 and the coil 8 and therefore, the carriage 6 is driven in the tracking direction by the electric current flowing to the coils 8 and 8. Also, the coils 12 and 12 are subjected to a vertical force by the same magnetic field by the electric current being supplied thereto and therefore can drive the objective lens 11 in the focusing direction.
In the above-described example of the prior art, however, the inner yokes 4 and 4 are passed through the lens holding member 10 and therefore, the rigidity of the lens holding member 10 becomes low, and this results in a reduction in the dynamic characteristic of focus control, which in turn is liable to cause sub-resonance. (FIG. 4 of the accompanying drawings shows the dynamic characteristic of focus control. Since the structural resonance of the lens holding member is as low as ten and several kHz, the focus controllable zone is narrow.) Also, the lens holding member 10 becomes bulky, and this leads to the disadvantage that the carriage 6 becomes bulky.